1. Field of the Invention
The present invention relates generally to helicopter rotor blades that are made from composite materials. More particularly, the present invention is directed to the processes and apparatus that are used in the manufacture of such composite rotor blades.
2. Description of Related Art
Rotor blades are a critical component of every helicopter. The rotor blades are subjected to a complex set of rather extreme aerodynamic forces that vary continually during flight. The rotor blades function as rotating airfoils or wings that are shaped to provide the aerodynamic lift required for a given aircraft. Rotor blades typically include a spar that extends from the root of the rotor blade to its tip. The spar is a major structural element of the rotor blade that provides the blade with the structural strength needed to carry high operational loads.
The typical rotor blade spar is a long tubular structure around which the rest of the blade is formed. The spar tube has an elliptical cross-section that is formed to provide a forward or leading edge and rearward or trailing edge. In order to provide optimum aerodynamic performance, many spar tubes include a slight twist about the longitudinal axis. Typical twists in the spar provide rotations of the elliptical cross-section of up to 10 degrees and more as one moves from the root of the rotor blade to its tip. In addition, the elliptical shape of the spar cross-section may be varied from the spar root to the spar tip to meet a variety of aerodynamic and structural loading parameters.
High strength materials, such as titanium and aluminum alloys, have typically been used to make rotor blades. These high strength metal materials are particularly well suited for forming the rotor blade spar. Titanium has been routinely formed into the relatively long, tubular spar structure and machined or otherwise fabricated to provide a complex variety of twists and varying cross-sectional shapes.
Composite materials have also been used to form rotor blade spars. The combination of light weight and structural strength have made composites a popular choice for making not only the rotor blade spar, but the entire rotor blade. Exemplary composite rotor blades and the processes for making them are described in U.S. Pat. Nos. 4,892,462; 5,346,367; 5,755,558; and 5,939,007.
The typical composite spar is fabricated by applying the uncured composite material to the surface of a long cylindrical mold or mandrel that is shaped to provide the interior surface of the spar tube. After the composite material is applied to the mold or mandrel, it is compacted and cured at an elevated temperature to provide the final spar structure. A problem associated with making composite spars revolves around what to do with the mold or mandrel once the spar has been formed. The length of the mold and the variations in elliptical cross-section of the spar, as well as any twist in the spar, make it very difficult to remove the mold or mandrel after the spar has cured.
One approach to solving the mold/mandrel removal problem has been to make a mold out of a material that is strong enough to maintain its shape during pre-cure fabrication of the composite spar, but which disintegrates or otherwise shrinks during the cure cycle so that it can be removed from the spar cavity or simply left in place. For example, a variety of foams have been used alone or in combination with an underlying hard mandrel structure to provide a suitable spar mold. The foam melts or otherwise shrinks to a fraction of its initial size during curing at elevated temperatures. The resulting shrunken mold is sufficiently small so that it can be removed from the spar cavity or left in place.
Although foam molds have been used successfully in fabricating composite spars for rotor blades, it is many times difficult to find a foam or other material that has the needed structural strength to maintain critical spar dimensions during formation of the spar, while at the same time being able to deteriorate relatively rapidly during cure. In addition, the mold can only be used once, which adds considerably to the cost of spar fabrication.